Mode-Locked Laser
Mode-locking is a technique in optics by which a laser can be made to produce pulses of light of extremely short duration, on the order of picoseconds (10−12 s) or femtoseconds (10−15 s).
The basis of the technique is to induce a fixed-phase relationship between the longitudinal modes of the laser's resonant cavity. The laser is then said to be “phase-locked” or ‘mode-locked.” Interference between these modes causes the laser light to be produced as a train of pulses. Depending on the properties of the laser, these pulses may be of extremely brief duration, as short as a few femtoseconds. The train of pulses is commonly in the 50-100 MHz repetition rate range.
Electro-Optic Modulator (EOM)
Electro-optic modulator (EOM) is an optical device in which a signal-controlled element exhibiting the electro-optic effect is used to modulate a beam of light. The modulation may be imposed on the phase, frequency, amplitude, or polarization of the beam.
The simplest kind of EOM consists of a crystal, such as lithium niobate, whose refractive index is a function of the strength of the local electric field. That means that if a lithium niobate crystal is exposed to an electric field, light will travel more slowly through it. But the phase of the light leaving the crystal is directly proportional to the length of time it takes for the light to pass through the crystal. Therefore, the phase of the laser light exiting an EOM can be controlled by changing the electric field applied to the crystal.
Combining this phase change with polarizers before and after the crystal, amplitude modulation can be achieved. When using an EOM as an amplitude modulator, the configuration is usually with two orthogonally aligned crystals. This helps reduce thermal drift. FIG. 1 shows an example setup of the EOM amplitude modulator.
The electro-optic amplitude modulator may be a Pockels cell type modulator consisting of two matched lithium niobate crystals 110, 120 packaged in a compact housing with an RF input connector. Applying an electric field to the crystal induces a change in the indices of refraction (both ordinary and extraordinary) giving rise to an electric field dependent birefringence which leads to a change in the polarization state of the optical beam. The Electro-optic crystal acts as a variable waveplate with retardance linearly dependent on the applied electric field. By placing a linear polarizer 140 at the exit, the beam intensity through the polarizer varies sinusoidally with linear change in applied voltage.
Electro-optic phase modulators provide a variable phase shift on a linearly polarized input beam. In one embodiment, the input beam is linearly polarized along the vertical direction which is the Z-axis of the crystal by a linear polarizer 130. A voltage at the RF input 150 is applied across the Z-axis electrodes 160 inducing a change in the crystal's extraordinary index of refraction thereby causing a phase shift in the optical signal.
DC Modulation
Two methods of DC control for amplitude modulation of mode-locked lasers are currently commonly used.
Existing techniques have achieved ˜DC-1 MHz modulation control using a high voltage/high power DC coupled linear amplifier. This method allows control to any output intensity level over the course of 10 to 100 laser pulses. FIG. 2 shows a schematic diagram of a linear amplifier.
Several other EOM drive manufacturers use a push-pull arrangement to switch between two slowly varying DC levels. This method allows for switching between two output intensity levels over the course of 2 to 3 laser pulses. Changing those DC levels takes about 1,000 to 10,000 laser pulses. This approach is very effective in edge blanking of an image or other applications where an on-off feature is needed. FIG. 3 shows a schematic diagram of a push-pull amplifier.
AC Modulation
Modulating the amplitude of a mode-locked laser with an AC waveform is commonly done. It is usually accomplished by adding circuitry to create a resonant tank between the EOM crystal (purely capacitive element) and other passive components. This resonant system can then be controlled by a relatively low power AC signal generator. Phase locking this system to the train of pulses from the mode-locked laser allows the overlay of a signal on the output intensity of the train of pulses. As shown in FIG. 4, an AC waveform 410 is applied to the output pulses 420 of a mode-locked laser, resulting in a modulated amplitude output 430.
The modulation techniques discussed above have the drawback that it takes many pulses to change the output amplitude from one value to another desired value. However, there is a need to increase the speed of modulation. In particular, a pulse-by-pulse control of the laser power would provide improvements of and open up many new uses of mode-locked lasers.